1. Field of the Invention
The present invention relates to a beam homogenizer which homogenizes a beam spot on a surface to-be-irradiated in a specified area, and further relates to a laser irradiation apparatus which irradiates a homogenized beam spot on a surface to-be-irradiated. Note that, a semiconductor device according to the present invention includes a display device such as an active matrix liquid crystal display device and an active matrix electro luminescence display device, an electro-optical device, and an electrical appliance utilizing such a display device or an electro-optical device, and the present invention also relates to a method of manufacturing the semiconductor device.
2. Description of the Related Art
Recently a technique of laser annealing that crystallizes or enhances crystallinity of an amorphous semiconductor film or a crystalline semiconductor film (a semiconductor film having crystallinity such as polycrystallinity or microcrystallinity except for single crystallinity) formed on an insulating substrate of glass or the like has widely studied. The semiconductor film is generally formed of a silicon semiconductor film.
In comparison with a quartz substrate that has been commonly used, a glass substrate is more economical, has higher workability and has the advantage that a substrate having a large area can be easily produced. This is the reason why the studies have been extensively conducted. A laser is preferably used for crystallization because the melting point of a glass substrate is low. The laser can impart high energy only to the non-single crystal semiconductor film without changing the temperature of the substrate greatly.
A crystalline silicon film formed by performing the laser annealing has a high mobility. Therefore, thin film transistors (TFTs) are formed by employing the crystalline silicon film, and they are generally used for, for example, a monolithic liquid-crystal electro-optical device in which the TFTs for pixels and for driver circuits are fabricated on one glass substrate. Since the crystalline silicon film is formed of a large number of crystal grains, it is called a “poly-crystalline silicon film” or a “poly-crystalline semiconductor film”.
Besides, a method wherein a beam from a pulsed laser beam of high power, such as an excimer laser, is processed by an optical system so as to define a tetragonal spot having a size of several cm square or a line having a length of at least 10 cm on a surface to-be-irradiated, and the projected position of the beam spot is scanned relatively to the surface to-be-irradiated, thereby carrying out the laser annealing, is preferably used since it has good productivity and is industrially superior.
When a linear beam spot is used, particularly, unlike in the case of using a punctiform beam spot that needs scanning in the forward, backward, right and left directions, the entire surface to-be-irradiated can be irradiated with the beam by scanning only in a direction perpendicular to the long side of the linear beam spot. This leads to high productivity. The linear beam spot here shall be a rectangular beam spot that has a high aspect ratio. Scanning is carried out in a direction perpendicular to the long side because it is the most efficient scanning direction. Because of the high productivity, the use of a linear beam spot obtained by processing a pulse excimer laser beam by an adequate optical system in laser annealing is becoming a mainstream in the manufacturing technology at present.
An example of an optical system for linearizing the shape of a beam spot (the sectional shape of a laser beam) on the irradiated surface is shown in FIG. 10. This optical system shown in FIG. 10 is a very general one. The aforementioned optical systems not only transform the sectional shape of the laser beam into a linear shape, but also homogenize the energy of the beam spot in the irradiated surface at the same time. Generally, an optical system that homogenizes the energy of a beam is referred to as a beam homogenizer. Namely, an optical system shown in FIG. 10 is also a beam homogenizer.
When the excimer laser, which is ultraviolet light, is used as a light source, the host material of the above-mentioned optical system may be entirely quartz, for example. The reason for using quartz resides in that a high transmittance can be obtained. Further, a coating with 99% or more transmittance on wavelength of the excimer laser that is used may preferably be employed.
FIG. 10A showing the side view will be explained first. A laser beam emitted from a laser oscillator 1201 splits the laser beam in one direction by cylindrical lens arrays 1202a and 1202b. The direction shall be referred to as a longitudinal direction here. When a mirror is incorporated midway of the optical system, the longitudinal direction will follow the direction of light reflected by the mirror. The beam spot of this structure is split into 4 beams. The split spots are then collected to 1 beam spot by a cylindrical lens 1204. Then, the spot is split again and reflected at a mirror 1207. Thereafter, the split spots are again collected to 1 beam spot at an irradiated surface 1209 by a doublet cylindrical lens 1208. A doublet cylindrical lens is a lens that is formed of two pieces of cylindrical lenses. Consequently, the energy in the longitudinal direction of the linear beam spot is homogenized and the length of the longitudinal direction of the linear beam spot is determined.
FIG. 10B showing the top view will be explained next. A laser beam emitted from the laser oscillator 1201 is split in a direction perpendicular to the longitudinal direction by a cylindrical lens array 1203. The perpendicular direction shall be referred to as a lateral direction here. When a mirror is incorporated midway of the optical system, the lateral direction will follow the direction of light reflected by the mirror. The beam spot in this structure is split into 7 beams. Thereafter, the laser beams split into 7 beams by the cylindrical lens 1205 are collected to 1 beam spot at the irradiated surface 1209. The steps shown with reference numerals 1207 to 1209 are drawn in broken lines; the broken lines show the positions of accurate light paths, lenses, and the irradiated surface in the case where the mirror 1207 is not arranged. Thus, the energy in the lateral direction of the linear beam spot is homogenized and the length of the lateral direction is also determined.
As described above, the cylindrical lens arrays 1202a, 1202b and 1203 work as lenses which split laser beams. Homogeneity of the energy distribution at the linear beam spot depends on the number of split beams.
The above lenses in the optical system are made of synthetic quartz responsive to an excimer laser. Further, the lenses are coated on the surfaces so that the excimer laser can transmit easily. Therefore, the transmittance of an excimer laser through one lens can be 99% or more.
The linear beam spot linearized with the above structure of the optical system is irradiated in an overlapping manner with gradual shifts to the lateral direction thereof. That allows to crystallize and to enhance the crystallinity of the entire surface of a non-single crystal silicon film by performing laser annealing.
A typical method of manufacturing a semiconductor film that is to be the object to be irradiated by a laser beam is shown next. First, a 5 inch square Corning 1737 substrate having a thickness of 0.7 mm is prepared as the substrate. Then a 200 nm-thick SiO2 film (silicon oxide film) is formed and a 50 nm-thick amorphous silicon film (hereinafter denoted by a-Si film) is formed on the surface of the SiO2 film. Both films are formed by employing the plasma CVD apparatus. The substrate is exposed under an atmosphere containing nitrogen gas at a temperature of 500° C. for 1 hour thereby reducing the hydrogen concentration in the film. Accordingly, resistance of the film to a laser is remarkably improved.
The XeCl excimer laser L4308 (wavelength: 308 nm, pulse width: 30 ns) manufactured by Lambda Co. is used as the laser oscillator. This laser oscillator generates a pulse oscillation laser and has the capacity to emit energy of 670 mJ/pulse. The spot size of the laser beam at the exit of the laser beam is 10×30 mm (both half-width). The exit of the laser beam is defined by the plane perpendicular to the direction the laser beam is traveling, immediately after the laser beam is emitted from the laser oscillator.
The shape of the laser beam generated by the excimer laser is generally rectangular and is expressed by an aspect ratio which falls under the range of the order of 1 to 5. The intensity of the laser beam spot indicates the Gaussian distribution stronger towards the center of the beam spot. The spot size of the excimer laser beam is transformed into a 125 mm×0.4 mm linear laser beam spot having the homogenous energy distribution by the optical system shown in FIG. 10.
When irradiating a laser beam to the above-mentioned semiconductor film, the most suitable overlapping pitch is approximately 1/10 of the short width (half-width) of the linear beam spot. The uniformity of the crystallinity in the semiconductor film is thus improved. According to the above example, the half-width of the linear beam spot was 0.4 mm, and therefore the pulse frequency of the excimer laser was set to 30 hertz and the scanning speed was set to 1.0 mm/s thereby irradiating the laser beam. At this point, the energy density at the irradiated surface of the laser beam was set to 450 mJ/cm2. The method described hitherto is a very general method employed for crystallizing a semiconductor film by using a linear beam spot.
Some conventional beam homogenizers utilize reflecting mirrors that can easily perform an accurate processing. (See Japanese Patent laid-open 2001-291681, for example.)
High processing accuracy is required for manufacturing the cylindrical lens arrays.
A cylindrical lens array is formed of cylindrical lenses arranged in a direction of curved lines. The direction of curved lines here shall refer to the perpendicular direction of the generatrix of the cylindrical surface. Cylindrical lenses forming a cylindrical lens array invariably have connected portions therebetween. The connected portions don't have curved surfaces; a beam irradiated through the connected portions transmits without being affected by the cylindrical lenses. The beam that reaches to the surface to-be-irradiated without the effect can cause the inhomogeneous energy distribution at the rectangular beam spot on the irradiated surface.
The cylindrical lenses forming the cylindrical lens array are necessarily fabricated with all the same accuracy. When the cylindrical lenses have different curvatures, laser beams split by the cylindrical lens array are not collected to the same position at an irradiated surface even by a condensing lens. Namely, the energy distribution at the rectangular beam spots on the irradiated surface is not homogenized
The inhomogeneous energy distribution at the beam spot on the irradiated surface is caused by a structural problem and processing accuracy of the cylindrical lens array forming an optical system. Namely, inhomogeneity arises from that all the laser beams induced into a homogenizer are not induced to the part where the cylindrical lenses function and that all the laser beams split by the cylindrical lens array are not collected to the same position.
Further, when the semiconductor film is crystallized by scanning a rectangular beam spot having the energy distribution in the direction of the short side of the rectangular on the irradiated surface indicating the gaussian distribution by the means described in the Related Art, a striped pattern in a direction perpendicular to the scanning direction appears clearly on the semiconductor film. The striped pattern synchronizes with inhomogeneous crystallinity of the semiconductor film. For example, the inhomogeneity emerges as a dispersion of electrical characteristics of a TFT, and the striped pattern is displayed on a panel employing the TFT.
The striped pattern is due to an instable output of a laser oscillator. Accordingly, the only way to eliminate the striped pattern is improving a laser oscillator. However, when the energy distribution in the direction of the short side of the rectangular beam spot on the irradiated surface is homogenized, the instability of the output of the laser oscillator is averaged and the striped pattern will fade into the background. Namely, the emergence of the striped pattern is inhibited. Accordingly, an optical system homogenizing the energy distribution has been required. Naturally, a rectangular beam spot with the homogeneous energy distribution can be obtained by employing a cylindrical lens array, however, a highly accurate optical system has been required.